Pregnancy Due Date Calculator
Estimates your due date, current pregnancy week, and trimester from your last menstrual period or conception date. Useful for expectant parents tracking milestones from the first positive test.
About this calculator
Pregnancy is typically 280 days (40 weeks) from the first day of the last menstrual period (LMP). The core formula adjusts for conception date and non-standard cycle lengths: daysRemaining = 280 − (startDate − (cycleLength − 28)). If you input a conception date instead of LMP, 14 days are added to align with standard gestational dating. A cycle shorter or longer than 28 days shifts the estimated ovulation day, so the calculator corrects by subtracting the cycle difference from the LMP-based count. The current gestational week is derived by dividing elapsed days by 7. Trimesters are divided at weeks 13 and 27. This method mirrors the Naegele's Rule used by most obstetric providers.
How to use
Suppose your LMP started 42 days ago and your average cycle is 30 days. Cycle difference = 30 − 28 = 2 days. Adjusted elapsed days = 42 − 2 = 40 days. Days remaining = 280 − 40 = 240 days. That places you at approximately Week 5 with about 240 days, or roughly 34 weeks, until your due date. If you instead know your conception date was 28 days ago, the calculator adds 14 days first: 28 + 14 = 42 elapsed days, then applies the same 280-day countdown.
Frequently asked questions
How does the pregnancy calculator determine my due date from my last menstrual period?
The calculator uses Naegele's Rule, which adds 280 days (40 weeks) to the first day of your last menstrual period. It then adjusts for your personal cycle length by subtracting the difference between your cycle and the standard 28-day cycle. This produces a personalized estimated due date rather than a one-size-fits-all result. Your healthcare provider may refine this estimate with an early ultrasound.
What is the difference between calculating from LMP versus conception date?
When you use LMP, gestational age is counted from the first day of your period, which is typically about two weeks before ovulation and conception. When you use conception date, the calculator adds 14 days to convert it to the equivalent LMP date before running the 280-day countdown. This ensures both methods produce the same due date estimate. The LMP method is the standard used in clinical obstetrics.
Why does cycle length affect the estimated due date?
Ovulation typically occurs around day 14 of a 28-day cycle, but women with longer or shorter cycles ovulate at different points. A woman with a 35-day cycle likely ovulates around day 21, meaning conception happens later than assumed by the standard calculation. The calculator corrects for this by shifting the due date forward or backward by the number of days your cycle differs from 28. Ignoring cycle length can result in an estimate that is off by a week or more.